Nocebo vs. placebo: the challenges of trial design in analgesia research.

The placebo effect in randomized clinical trials appears to have increased thereby contributing to problems of demonstrating statistically reliable effects of treatments that directly target biological mechanisms. The shortcomings of randomized clinical trials are currently discussed along with potential improvements of trial designs. In this review we explain how utilizing knowledge from the placebo and nocebo mechanisms literature could improve the information that can be obtained from randomized clinical trials. We present three major challenges in randomized clinical trials: (i) increasing placebo effects, (ii) variability of the placebo effect, and (iii) risk of un-blinding. We then explain how recent placebo and nocebo studies of effects of verbal suggestion, expectancy, and emotions may improve understanding and discussion of increasing placebo effects, account/control for large parts of the variability of placebo effects, and suggest ways to improve blinding in future trials.